Excessively hard A48 Class 30

[geesamand]

This may be a really basic question (I only play a materials guy on TV). We recently finished a run of cast A48 Class 30 and the machining was extremely difficult. I checked the standard and ASTM A48 does not specify a limit on ultimate strength and hardness. Our company has used this material for many years and this might be a first for us.

Where might I begin to troubleshoot this issue? Is there an affordable way to control this problem without changing material spec?

Dave

 

[mk1979]

that,s right the machining of this grade of cast iron may be less machinable,the P content is 0.15%. but you cann,t make any heat treatment process to get thi smaterial machinable.just you can replace the machining tip with other hard one.

 

[TVP]

This is an excerpt from Section 6 (my emphasis):

6.1 When agreed upon in writing between the manufacturer
and the purchaser, it may be necessary for the castings to meet
special requirements as to hardness, chemical composition,
microstructure, pressure tightness, radiographic soundness,
dimensions, surface finish, and so forth.

It is very common to specify hardness ranges and microstructure that are most conducive to machining processes. Here is an example of a more descriptive requirement:

Hardness: 187-241 HB
Microstructure: Type A graphite, size 3-5, in a matrix of pearlite. Free ferrite = 5% maximum. The presence of cementite or ferrite network shall constitute reason for rejection.

 

[arunmrao]

Agree with TVP. Gray iron castings must be machinable,unless there is a case of poor inoculation, presence of Chromium, or thin section .

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[swall]

To add to what TVP stated, it is important to emphasize that the hardness requirement applies to the actual castings, not separately cast bars.

 

[geesamand]

Thanks everyone. I think we're on the trail now and I'm relieved to know that a simple hardness check may suffice to control this issue in the future.

I found an old company grey iron spec that imposes the additional requirement of 160-220Bhn, and our vendor confirmed that holding hardness in this range this is normal practice for this material.

I'm starting by tracing down heat numbers, testing hardness of finished parts, and reviewing whether the premachining stress relief procedure could have played a role.

David